Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating seismic waves.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are essentially impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources are airguns which produce a high amount of acoustics energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the airgun propagate in all directions. A typical frequency range of the emitted acoustic waves is between 6 and 300 Hz. However, the frequency content of the impulsive sources is not fully controllable and different sources are selected depending on the needs of a particular survey. In addition, the use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources may be used are vibratory sources. Vibratory sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations. However, there is no large scale use of such sources as they have limited power and are not reliable due to the number of moving parts required to generate the seismic waves. A positive aspect of vibratory sources is that they can generate signals that include various frequency bands, commonly referred to as “frequency sweeps”. In other words, the frequency band of such sources may be controlled better compared to impulsive sources. However, the known vibratory sources do not provide a high vertical resolution as the typical frequency range of a marine seismic source represents approximately four octaves. A few examples of such sources are now discussed.
U.S. Pat. No. 7,551,518, the entire content of which is incorporated herein by reference, describes a flextensional source. The source includes a combination of actuators having a flexure system that acts as a force transformer. A disadvantage of such a system is the large size required to fit the flexible elements in order to prevent overstressing them. The reliability of such system may also be a weak point as those flexible parts are constantly subjected to deformations during sound emission.
International Patent WO 8603015, the entire content of which is incorporated herein by reference, describes a seismic generator having a hydraulically driven piston source. However, the use of a pressurized hydraulic fluid to drive the piston might be a threat to the environment in case of a leak.
U.S. Pat. No. 6,464,035, the entire content of which is incorporated herein by reference, describes a towable marine seismic energy source for creating intense swept-frequency and pulse-coded signals in a body of water. The fish-like source is half way between a pure vibrator and an impulse source. The source is driven by a system that uses a pressurized hydraulic fluid. Thus, there is a risk of polluting the environment. The source appears to be a weak sweep source because of stroke limitations, which is undesirable as a good source needs to generate low frequency sweeps.
Thus, based at least on the above noted deficiencies of the existing sources there is a need to develop new sources that will eliminate or minimize their impact on the environment, also be more reliable, have an extended lifetime, and increase the vertical resolution by adding at least another octave. Accordingly, it would be desirable to provide systems and methods that overcome the afore-described problems and drawbacks.